This application is based on application Ser. No. 2000-90690 filed in Japan, the content of which is hereby incorporated by reference herein in its entirety.
The present invention relates to an optical device for splitting and combining beams of light and a method of manufacturing the optical device. More particularly, the invention relates to a photonic crystal device comprising a photonic crystal layer formed in a layer where a plurality of media with different refractive indices are periodically arranged and a method of manufacturing such a device.
A conventionally designed prior art optical functional device having a photonic crystal where a plurality of media with different refractive indices are periodically arranged has a structure as shown in FIG. 6. In the prior art optical functional device 1, an optical waveguide layer 2 is laminated on a clad layer 3. The clad layer 3 comprises a medium having a lower refractive index than the optical waveguide layer 2. Above the optical waveguide layer 2 in the figure is an air layer 8 having a lower refractive index than the optical waveguide layer 2. Since the optical waveguide layer 2 is thus sandwiched between media with lower refractive indices, the light incident on the optical waveguide layer 2 is redirected while being trapped in the optical waveguide layer 2.
The optical waveguide layer 2 comprises a photonic crystal 2 where media 2a and 2b with different refractive indices are periodically arranged. For example, as shown in FIG. 6, a photonic crystal 2 where a cylindrical medium 2a comprising air and a medium 2b comprising silicon are two-dimensionally periodically arranged can be formed by defining vacancies 2a in a silicon thin film 2b at predetermined intervals.
Photonic crystals have a characteristic that causes anisotropy of refractive index dispersion. By appropriately selecting the refractive indices of the media 2a and 2b, the shapes of the media 2a and 2b such as a cylinder or a prism, the kind of the grating such as a triangular grating or a square grating, and the period of the arrangement, different optical characteristics can be obtained for light beams of desired wavelengths and polarization directions.
By doing this, for example, light beams of wavelengths xcex1 and xcex2 incident from the same direction can be made to exit in different directions as shown in FIG. 6. Such devices can also be adapted to make light beams of different wavelengths being incident from different directions exit in the same direction. Moreover, photonic crystal devices can be adapted to reflect a light beam of a specific wavelength. By employing this characteristic, photonic crystals can be used as light signal splitters and combiners, or filters.
As demonstrated in FIGS. 7(a)-7(g), the optical functional device 1 of the prior art is manufactured by a process requiring numerous steps. Referring first to FIG. 7(a), a substrate 11 serves as the claim layer 3 of FIG. 6. Referring to FIG. 7(b), to that layer, a film of a medium 12 of a material such as silicon is formed. Then, as shown in FIG. 7(c), a resist layer 13 is applied to the medium 12. As shown in FIG. 7(d), the resist layer 13 is formed into a predetermined periodic pattern or shape.
Referring now to FIG. 7(e), periodically arranged concave portions 12a defined through the medium 12 may be formed by a method such as etching. By removing the resist layer 13, as shown in FIG. 7(f), a photonic crystal is formed where air in the concave portions 12a and the medium 12 are two-dimensionally periodically arranged.
Referring to FIG. 7(g), by filling the concave portions 12a with a medium 14, a photonic crystal is obtained where the medium 12 and the medium 14 are two-dimensionally periodically arranged and have different optical characteristics. Consequently, the optical functional device 1 is obtained where the clad layer 3 shown in FIG. 6 comprises the substrate 11 and the media 2a and 2b comprise the media 12 and 14.
Alternately, optical functional devices of the prior art as shown in FIG. 6 may be manufactured wherein the optical waveguide layer 2 comprises a periodic porous material formed by a method such as anodic oxidation and is bonded or fused to the clad layer 13.
However, according to the prior art conventional method of forming a photonic crystal device 1, since the optical waveguide layer 2 is integrated with the clad layer 3 by laminating or bonding it to the clad layer 3, the number of manufacturing steps and therefore the number of man-hours is large, thereby increasing the cost. In addition, the optical waveguide layer 2 and the clad layer 3 may be readily and undesirably separated, which decreases the yield.
An object of the present invention is to provide an improved photonic crystal device.
Another object of the present invention is to provide a photonic crystal device wherein cost reduction is achieved by reducing the number of man-hours required to form the device.
Yet another object of the present invention is to provide a photonic crystal device which has inseparable layers thereby improving the yield.
These objects are achieved by a multi-layer photonic crystal device comprising a medium which is commonly and integrally formed in at least two layers of the device. The photonic crystal device of the first embodiment comprises a first medium having a thickness and periodically defining a plurality of cylindrically-shaped concave portions throughout. The concave portions have a depth which is less than the thickness of the first medium. A second medium, preferably having a higher refractive index than the first medium, is filled in the concave portions. This creates a first layer comprising a mixed media of cylinders of the second medium periodically interspersed with the first medium forming a photonic crystal. Depending on the average index of refraction of the first layer, the photonic crystal layer may or may not also be an optical waveguide layer. In the preferred embodiment where the second medium has a higher index of refraction than the first medium, this mixed media first layer (photonic crystal) is the optical waveguide layer which results from the periodically arranged first medium and second medium of the first layer due to the different indices of refraction between the two media.
In the preferred embodiment, a second layer of the photonic crystal device, a layer adjacent the mixed media first layer, is entirely formed from the first medium. Since the refractive index of the first medium is lower than that of the second medium, the first layer has an overall higher average refractive index than the second layer. The first layer, which forms the optical waveguide layer, is disposed between the second layer and a layer of air, which also has a lower refractive index than the first layer. Consequently, the light incident on the first (optical waveguide) layer can be redirected while being trapped in the optical waveguide layer.
The photonic crystal device is preferably manufactured by a process wherein a resist layer is applied to a first medium, which is preferably formed as a consistent thickness, consistent material structure, in a resist layer applying step. Then, the resist layer is formed into a predetermined periodic pattern or shape in a patterning step by removing portions of the resist layer. Portions of the first medium corresponding to the periodic pattern of the resist layer are then removed in a concave portion defining step. The resist layer is then removed in a resist layer removing step. Finally, the second medium, preferably having a different index of refraction than the first medium, is filled in the concave portions and any of the second medium outside of or overflowing from the concave portions is removed in a second medium removing step. Alternately, the resist layer may be removed after the filling step, thereby eliminating the need for the final second medium removing step.